Hydraulic transaxle

ABSTRACT

An integrated hydraulic transaxle (IHT) comprises a hydrostatic transmission (HST), an axle driven by the HST, and a housing incorporating the HST and the axle. The HST includes a variable displacement hydraulic pump driven by a prime mover, and a hydraulic motor fluidly connected to the hydraulic pump. The hydraulic pump has a trunnion type movable swash plate. The internal displacement of the hydraulic motor is larger than the internal displacement of the hydraulic pump so as to increase the hydraulic deceleration activity of the HST. In the IHT, a mechanical deceleration drive train is interposed between the hydraulic motor and the axle. The mechanical deceleration activity of the mechanical deceleration drive train is reduced so far as the increase of the hydraulic deceleration activity of the HST.

CROSS-REFERENCE TO RELATED APPLICATION

The present application is a Continuation of application Ser. No.10/883,729, filed Jul. 6, 2004 now U.S. Pat No. 7,043,908, which isincorporated in its entirety herein by reference thereto.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to an integrated hydraulic transaxle (IHT); i.e.,IHT incorporating a hydrostatic transmission (HST) and an axle, andpreferably, a mechanical deceleration drive train interposed between theHST and the axle.

2. Related Art

There is a well-known conventional hydraulic transaxle, i.e., IHT, whichhas a transmission casing integrally incorporating an HST, an axle, anda mechanical deceleration drive train (gearing) interposed between theHST and the axle. The HST comprises a hydraulic pump and motor fluidlyconnected to each other, wherein the hydraulic pump and/or motor has amovable swash plate for changing its displacement.

Reduction of the deceleration ratio of a mechanical deceleration drivetrain in an IHT is advantageous in minimizing and simplifying the IHTand reducing noise generated from the IHT. If the deceleration ratio ofthe mechanical deceleration drive train in an IHT is reduced, the HST isrequested to increase its deceleration ratio between the hydraulic pumpand motor so as to compensate for the reduction of deceleration ratio ofthe mechanical deceleration drive train. U.S. Pat. No. 5,201,692, forexample, discloses a conventional IHT having an HST, in which theinternal displacement of hydraulic pump is smaller than the internaldisplacement of hydraulic motor so as to increase the deceleration ratioof the HST.

In a usual HST, internal displacements of the pump and motor are equalto each other so that the maximum rotary speeds of the pump and motorare equal to each other. In this state, reciprocation of pistons in acylinder block of the pump is synchronous to reciprocation of pistons ina cylinder block of the motor, thereby generating large vibration andnoise. In the HST disclosed in the document, the difference of internaldisplacement between the hydraulic pump and the motor, such as not tosynchronize the rotational frequency of the pump to that of the motor,is also advantageous to reduce vibration and noise.

In the IHT disclosed in the above document, the axial piston typehydraulic pump is provided with a cradle type movable swash plate. Thecradle type movable swash plate has an arcuately convex surface slidablyfitting to an arcuately concave inner surface of the IHT housing or aretainer fitted therealong. The concave surface of the IHT housing orthe retainer must be so long as to ensure the sufficient slide range ofthe swash plate, thereby expanding the IHT housing. Further, the concavesurface must be processed to resist strain, thereby increasing themanufacturing cost of the IHT. The fitting of surfaces between the swashplate and the IHT housing is subject to contamination and causesfrictional resistance to reduce the efficiency of operational force formoving the swash plate. Thus, the disclosed IHT involves some obstaclesin the way of being reduced in size and costs while ensuring itsdurability.

Furthermore, in the IHT disclosed in the above document, the hydraulicmotor extends a motor shaft relatively rotatably penetrating a centersection on which the hydraulic pump and motor are mounted and fluidlyconnected to each other through a closed fluid circuit therein. Theperipheral surface area of the motor shaft slidably rotatably contactingthe center section or a thrust bearing therebetween is considerablylarge so as to reduce the efficiency of transmitting power from thehydraulic motor to the motor shaft.

Such a loss of transmission efficiency is disadvantageous to minimizethe HST because the HST must have such a large displacement as to absorbthe loss.

SUMMARY OF THE INVENTION

A first object of the present invention is to provide an IHT, i.e., anintegrated hydraulic transaxle comprising a housing incorporating ahydrostatic transmission (HST) and an axle driven by the HST, whereinthe HST is improved to facilitate for its reduction of noise andvariation, and means for changing variable displacement of a hydraulicpump of the HST is improved to enhance the durability and operationalefficiency thereof while being reduced in size and costs.

To achieve the object, in a first aspect of the IHT of the presentinvention, the HST comprises a variable displacement hydraulic pumpdriven by a prime mover and a hydraulic motor fluidly connected to thehydraulic pump, wherein the hydraulic pump is provided with a trunniontype movable swash plate, and wherein an internal displacement of thehydraulic motor is different from an internal displacement of thehydraulic pump. The difference of internal displacement between thehydraulic pump and motor such as not to synchronize the rotationalfrequency of the motor to that of the pump is advantageous to reducevibration and noise. The trunnion type movable swash plate does not havea surface slidably contacting a surface of another member such as aretainer or a housing, thereby being efficiently movable andfacilitating for the IHT in reduction of size and costs and inenhancement of durability.

Preferably, the internal displacement of the hydraulic motor is largerthan the internal displacement of the hydraulic pump so as to increasethe hydraulic deceleration activity of the HST. The increase ofhydraulic deceleration activity, i.e., deceleration ratio of the HSTfacilitates for reduction of components, such as gears, of a mechanicaldeceleration drive train interposed between the HST and the axle in sizeor number.

To make the internal displacement of the hydraulic motor larger than theinternal displacement of the hydraulic pump, preferably, each of thehydraulic pump and motor comprises a cylinder block having cylindricalbores, and pistons reciprocally fitted into the respective cylindricalbores of the cylinder block, wherein the total volume of the cylindricalbores of the hydraulic motor is larger than the total volume of thecylindrical bores of the hydraulic pump.

While the internal displacement of the hydraulic motor is larger thanthe internal displacement of the hydraulic pump so as to increase thehydraulic deceleration activity of the HST, preferably, the IHT furthercomprises a mechanical deceleration drive train interposed between thehydraulic motor and the axle, wherein the mechanical decelerationactivity of the mechanical deceleration drive train is reduced so far asthe increase of the hydraulic deceleration activity of the HST. Thereduction of the mechanical deceleration activity, i.e., decelerationratio of the mechanical deceleration drive train is directed toreduction of components thereof in size or number, thereby facilitatingfor reduction of noise, size and costs of the IHT.

While the internal displacement of the hydraulic motor is larger thanthe internal displacement of the hydraulic pump so as to increase thehydraulic deceleration activity of the HST, preferably, a motor shaft isinterposed between the hydraulic motor and the mechanical decelerationdrive train, wherein an end surface of the motor shaft is disposedbetween both end surfaces of the cylinder block.

A second object of the present invention is to provide an IHT comprisinga housing incorporating an HST and an axle, wherein the HST is improvedto facilitate for its reduction of noise and variation, and a motorshaft of a hydraulic motor of the HST is improved to enhance theefficiency of transmitting power to the axle (through a mechanicaldeceleration drive train).

To achieve the second object, in a second aspect of the IHT of theinvention, the HST comprises a variable displacement hydraulic pumpdriven by a prime mover, and a hydraulic motor fluidly connected to thehydraulic pump, the hydraulic motor including a cylinder block and amotor shaft, wherein an end surface of the motor shaft is disposedbetween both end surfaces of the cylinder block, and wherein an internaldisplacement of the hydraulic motor is different from an internaldisplacement of the hydraulic pump. The difference of internaldisplacement between the hydraulic pump and motor such as not tosynchronize the rotational frequency of the motor to that of the pump isadvantageous to reduce vibration and noise. The arrangement of the endsurface of the motor shaft between both of end surfaces of cylinderblock is directed to shortening of the motor shaft so as to enhance theefficiency of transmitting power from the cylinder block to the motorshaft, thereby facilitating for minimization of the HST with thehydraulic pump and motor having the different internal displacements.

Preferably, the internal displacement of the hydraulic motor is largerthan the internal displacement of the hydraulic pump so as to increasethe hydraulic deceleration activity of the HST. The increase ofhydraulic deceleration activity, i.e., deceleration ratio of the HSTfacilitates for reduction of components, such as gears, of a mechanicaldeceleration drive train interposed between the HST and the axle in sizeor number.

To make the internal displacement of the hydraulic motor larger than theinternal displacement of the hydraulic pump, preferably, each of thehydraulic pump and motor comprises a cylinder block having cylindricalbores, and pistons reciprocally fitted into the respective cylindricalbores of the cylinder block, wherein the total volume of the cylindricalbores of the hydraulic motor is larger than the total volume of thecylindrical bores of the hydraulic pump.

While the internal displacement of the hydraulic motor is larger thanthe internal displacement of the hydraulic pump so as to increase thehydraulic deceleration activity of the hydrostatic transmission,preferably, the IHT further comprises a mechanical deceleration drivetrain interposed between the motor shaft and the axle, wherein themechanical deceleration activity of the mechanical deceleration drivetrain is reduced so far as the increase of the hydraulic decelerationactivity of the HST. The reduction of the mechanical decelerationactivity, i.e., deceleration ratio of the mechanical deceleration drivetrain is directed to reduction of components thereof in size or number,thereby facilitating for reduction of noise, size and costs of the IHT.

These, further and other objects, features and advantages will appearmore fully from the following description with reference to accompanyingdrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a plan view partly in section of an integrated hydraulictransaxle (IHT) from which an upper housing member is removed accordingto the present invention.

FIG. 2 is a sectional side view of the above.

FIG. 3 is a sectional side view of the above showing a braking system.

FIG. 4 is a sectional side view of the above when provided with anotherreservoir tank.

FIG. 5 is an enlarged fragmental plan view partly in section of the IHTwith the upper housing member removed, showing the basic structure ofsupporting a shortened motor shaft in a hydraulic motor.

FIG. 6 is a sectional plan view of the hydraulic motor having the basicstructure of supporting the shortened motor shaft, wherein a splineportion of the motor shaft is reshaped so as to have a guide portion.

FIG. 7 is an enlarged fragmental plan view partly in section of the IHTwith the upper housing member removed, showing a reshaped structure ofsupporting the shortened motor shaft in the hydraulic motor.

FIGS. 8 to 26 are sectional plan views of the hydraulic motors havingvarious structures of supporting the shortened motor shafts and cylinderblocks.

DETAILED DESCRIPTION OF THE INVENTION

An IHT (integrated hydraulic transaxle) 1 will be described mainly withreference to FIGS. 1 and 2.

IHT 1 comprises a housing 2 constituted by upper and lower divisionalhousing members 2 a and 2 b joined to each other through a horizontaljoint surface. Left and right lateral axles 4 are supported by upperhousing member 2 a above the horizontal joint surface. Alternatively,housing 2 may be constituted by left and right divisional members joinedto each other through a vertical surface perpendicular to axles 4.

Housing 2 incorporates an HST (hydrostatic transmission) 3, a mechanicaldeceleration drive train (deceleration gearing) 50, a differentialgearing 60 and a braking system 70. A pump displacement controlling unit40 is assembled to housing 2.

HST 3 comprises a hydraulic pump 10, a hydraulic motor 20, and a centersection 30, wherein hydraulic pump 10 and motor 20 are mutually fluidlyconnected through center section 30. Hydraulic motor 20 having ahorizontal motor shaft 21 is mounted onto a vertical surface of centersection 30 so as to arrange motor shaft 21 in parallel to axles 4, andhydraulic pump 10 having a vertical pump shaft 11 is mounted upright oncenter section 30 so as to be disposed between axles 4 and hydraulicmotor 20. Differential gearing 60 is disposed around proximal endportions of axles 4 so as to differentially connect axles 4 to eachother. Deceleration gearing 50 is interposed between motor shaft 21 anddifferential gearing 60 beside HST 3 along one of left and right sidewalls of housing 2 (in FIG. 1, along the left side wall). An internalpartition 2 c made of housing 2 separates a chamber for HST 3 from achamber for deceleration gearing 50, differential gearing 60, brakingsystem 70 and axles 4.

Pump shaft 11 of hydraulic pump 10 projects upward from housing 2 so asto serve as an input shaft fixedly provided thereon with an input pulley6 and a cooling fan 7. Input pulley 6 is drivingly connected to anoutput pulley of an engine via a belt.

Variable displacement hydraulic pump 10 comprises a cylinder block 12,pistons 13, and a trunnion type movable swash plate 14. Cylinder block12 is provided therein with an axial center hole into which pump shaft11 is not-relatively rotatably fitted. Cylinder block 12 is fartherprovided therein with a plurality of piston holes 12 a around the axialcenter hole in parallel.

Pistons 13 are reciprocally fitted into respective piston holes 12 athrough respective biasing springs. Swash plate 14 is provided with athrust bearing 15 against which heads of pistons 13 are pressed. Swashplate 14 has a central opening through which pump shaft 11 is freelyrotatably passed.

Pump displacement controlling unit 40 is operatively connected to swashplate 14 so as to tilt swash plate 14, thereby adjusting thedisplacement of hydraulic pump 10, i.e., the amount of oil deliveredfrom pump 10 to motor 20, and changing the direction of oil flow betweenpump 10 and motor 20.

Upper housing member 2 a has an outward opening covered with a sidecover 2 d in its side wall (in FIG. 1, the right side wall) laterallyopposite to deceleration gearing 50. Pump displacement controlling unit40 is an assembly including side cover 2 d, a pair of coaxial trunnionshafts 42 and a speed control lever 41, assembled together with swashplate 14. As shown in FIG. 2, swash plate 14 has a pair of downwardlyextended opposite tab portions 14 a to which respective trunnion shafts42 are fixed. One of trunnion shafts 42 (first trunnion shaft 42) isjournalled by side cover 2 d, and fixedly provided on an outer endportion thereof with speed control lever 41. A neutral returning spring44 is wound around first trunnion shaft 42 between swash plate 14 andside cover 2 d. Movable swash plate 14 has a pin 45 projecting inparallel to trunnion shaft 42. An eccentric shaft 46 parallel totrunnion shaft 42 is passed through side cover 2 d and screwed uptogether with side cover 2 d by an adjuster nut 43 provided thereonoutside side cover 2 d. Both ends of spring 44 are extended so as topinch pin 45 and an inner end portion of eccentric shaft 46.

When pump displacement controlling unit 40 is assembled to housing 2,swash plate 14 is inserted into housing 2 through the side opening ofupper housing member 2 a, the other trunnion shaft (second trunnionshaft) 42 is rotatably supported into partition 2 c, and side cover 2 dis disposed to cover the side opening of upper housing member 2 a andfastened to upper housing member 2 a with bolts. In pump displacementcontrolling unit 40 completely assembled to housing 2, speed controllever 41 fixed on first trunnion shaft 42 is disposed outside housing 2so as to be operatively connected to a speed controlling device on avehicle. Eccentric shaft 46 can be rotated by screwing adjuster nut 43thereon outside housing 2 so as to be correctly positioned at theproximal end portion thereof pinched by spring 44 in correspondence tothe neutral position of swash plate 14.

By rotating speed control lever 41, trunnion shafts 42 are rotatedtogether with swash plate 14 so as to change the displacement of pump 10or change the oil delivery direction of pump 10. Pin 45 on swash plate14 pushes one extended end of spring 44 apart from the other extendedend of spring 44 retained by eccentric shaft 46, thereby generating abiasing force for returning swash plate 14 to the neutral position.

As mentioned above, pump displacement controlling unit 40 includingswash plate 14 is an assemble facilitating to be easily attached ordetached to and from IHT 1. In comparison with a cradle type movableswash plate, trunnion type swash plate 14 does not require such acomplicated processing of housing 2 as to provide a slide surfacecontacting swash plate 14 required by the cradle type swash plate,thereby saving costs, and being free from contaminant obstructive to themovement and neutral-location of the swash plate.

Center section 30 is fastened to housing 2 by bolts so as to be spacedat the bottom surface thereof from an inner bottom surface of housing 2.An oil filter 31 is interposed between the bottom surface of centersection 30 and the inner bottom surface of housing 2. A pair of oilducts 32 and 33 are bored in center section 30 so as to circulate oilbetween hydraulic pump 10 and motor 20 mounted on center section 30. Apair of check valves 34 are disposed in center section 30 so as to allowoil to flow into respective oil ducts 32 and 33 through oil filter 31from an oil sump in housing 2.

Fixed displacement hydraulic motor 20 comprises a cylinder block 22,pistons 23, and a fixed swash plate 24. Cylinder block 22 is providedtherein with an axial center hole into which motor shaft 21 isnot-relatively rotatably fitted. Cylinder block 22 is further providedtherein with a plurality of piston holes 22 a around the axial centerhole in parallel. Pistons 23 are reciprocally fitted into respectivepiston holes 22 a through respective biasing springs. Swash plate 24 isprovided with a thrust bearing 25 against which heads of pistons 23 arepressed. Swash plate 24 has a central opening through which motor shaft21 is freely rotatably passed.

In a usual HST, internal displacements of pump and motor are equal toeach other so that the maximum rotary speeds of the pump and motor areequal to each other. In this state, reciprocation of pistons in acylinder block of the pump is synchronous to reciprocation of pistons ina cylinder block of the motor, thereby generating large vibration andnoise.

In HST 3, according to the present embodiment, the internal displacementof pump 10 is different from that of motor 20, so as not to synchronizethe rotational frequency of pump 10 to that of motor 20, therebyreducing vibration and noise.

More specifically, the internal displacement of motor 20 is set largerthan the internal displacement of pump 10. The total volume of pistonholes 12 a or 22 a defines the internal displacement of each of pump 10and motor 20. The amount of oil flow through each of pump 10 and motor20 is determined in proportion to the total volume of piston holes 12 aor 22 a.

While the number and axial lengths of piston holes 22 a in motorcylinder block 22 are equal to those of piston holes 12 a in pumpcylinder block 12, a diameter L2 of each piston hole 22 a is longer thana diameter L1 of each piston hole 12 a so that the internal displacementof motor 20 is larger than that of pump 10. Of course, pistons 23 arediametrically larger than pistons 13, so as to correspond to such pistonholes 22 a.

Alternatively, the fixed angle of swash plate 24 may be increased so asto increase the stroke of pistons 23, or piston holes 22 a may beincreased in number to be more than piston holes 12 a. Furthermore,washers may be inserted in respective piston holes 12 a so as to reducethe total volume of piston holes 12 a to a level less than that ofpiston holes 22 a. Any manner may be used for making the internaldisplacement of motor 20 larger than that of pump 10.

It is assumed that, when hydraulic pump 10 (pump shaft 11) is rotated ata constant speed, and its effective displacement is set to a fixed value(its movable swash plate is fixed to a certain position), hydraulic pump10 supplies oil to a conventional hydraulic motor, whose internaldisplacement is equal to pump 10, and hydraulic motor 20, whosedisplacement is larger than that of each of hydraulic pump 10 and of theconventional hydraulic motor, respectively. The rotary speed of motor 20(motor shaft 21) becomes smaller than that of the conventional motor(its motor shaft). In other words, the deceleration ratio betweenhydraulic pump 10 and motor 20 (the rotational speed ratio of pump shaft11 to motor shaft 21) is larger than that between hydraulic pump 10 andthe conventional hydraulic motor. Further specifically, the hydraulicdeceleration activity of HST 3, in which the internal displacement ofmotor 20 is larger than that of pump 10, is enhanced in comparison withthe hydraulic deceleration activity of a conventional HST in which thehydraulic pump and motor have an internal displacement equal to that ofpump 10.

Deceleration gearing 50 comprises gears 51, 53 and 54 and a countershaft 55. Gear 51 is fixed on a portion of motor shaft 21 projectinginto the chamber for deceleration gearing 50 from the chamber for HST 3through partition 2c. Counter shaft 55 is rotatably supported by anouter side wall and partition 2c of housing 2 and disposed in parallelbetween motor shaft 21 and axles 4. Gear 53, which is diametricallylarger than gear 51, is fixed on counter shaft 55 so as to constantlymesh with gear 51. Differential gear 60 has a bull gear 61. Gear 54,which is diametrically smaller than bull gear 61, is fixed on countershaft 55 adjacent to gear 53 so as to constantly mesh with bull gear 61.

Differential gearing 60 is provided with a differential locking system62. Differential locking system 62 comprises a pushpin 63 and a lockingmember 64. Pushpin 63 is disposed in parallel to axles 4 and projects ata distal end out of housing 2. By pushing the distal end of pushpin 63outside housing 2, pushpin 63 is axially moved and switched between alocking position and an unlocking position. Locking member 64 is axiallyslidably fitted on one axle 4 (in FIG. 1, right axle 4) and connected topushpin 63 so as to axially move integrally with pushpin 63. Lockingmember 64 has a claw 64 a constantly inserted into a hole 61 a bored inbull gear 61 so as to rotate integrally with bull gear 61. Indifferential gearing 60, a differential side gear 65 fixed on (right)axle 4 is formed with recesses 65 a. Locking member 64 has clutch teeth64 b.

When pushpin 63 is set in the locking position, clutch teeth 64 b arefitted into respective recesses 65 a so as to lock (right) axle 4 tobull gear 61 through locking member 64, thereby locking axles 4 to eachother. When pushpin 63 is set in the unlocking position, clutch teeth 64b are disposed out of recesses 65 a so as to separate (right) axle 4from locking member 64, thereby ensuring the differential connection ofaxles 4.

Referring to FIGS. 1 and 3, braking system 70 is constructed around anend portion of motor shaft 21 adjacent to gear 51. Brake system 70comprises a brake arm 71, a camshaft 72, a pressure plate 73 and a brakedisk 74. A top portion of vertical camshaft 72 projects upward fromhousing 2 so as to be fixedly provided thereon with a brake arm 71operatively connected to a braking operation device (such as a brakepedal) on a vehicle. Camshaft 72 has a sectionally semicircular camportion contacting pressure plate 73. Brake disk 74 is fixed on the endportion of motor shaft 21. Pressure plate 73 is disposed between brakedisk 74 and camshaft 72. By rotating brake arm 71 and camshaft 72, thecam portion of camshaft 72 pushes and moves pressure plate 73, wherebypressure plate 73 is pressed against brake disk 74 so as to brake motorshaft 21. A brake pad 75 is disposed between brake disk 74 and the sidewall of housing 2 so as to cushion brake disk 74 when braked.

In IHT 1 having the above structure, the engine power (rotational forceof input shaft (pump shaft) 11) while transmitted to axles 4 isdecelerated by the hydraulic deceleration activity of HST 3 and themechanical deceleration activity of deceleration gearing 50.

The driving of gears of a deceleration gearing between an HST and axlesis one of major cause for noise generated from an IHT. Reduction ofengine speed may be taken into account to reduce the noise. However, inIHT 1, the mechanical deceleration activity, i.e., the decelerationratio of deceleration gearing 50 is reduced by reducing the gearsthereof in size or in number, thereby reducing noise generated from IHT1. Such reduction of the gears in size or number is also advantageous inreduction of costs for making IHT 1.

As mentioned above, the hydraulic deceleration activity, i.e., thedeceleration ratio of HST 3 is increased so as to compensate for thereduction of the mechanical deceleration activity of decelerationgearing 50, thereby ensuring a sufficient torque for driving axles 4.Conversely speaking, the enhance of the hydraulic deceleration activityof HST 3, which causes the reduction of noise from HST 3 itself, alsoenables the reduction of the mechanical deceleration activity ofdeceleration gearing 50.

Referring to FIGS. 1, 2 and 4, variation of reservoir tanks applied toIHT 1 will now be described.

Referring to FIG. 1, a reservoir tank 80 or 180 is mounted upright onthe top of housing 2 above axles 4 and differential gearing 60 so as toabsorb the expansion of oil sump in housing 2 caused by heat from actingHST 3. When viewed in plan, a portion of reservoir tank 80 or 180,adjacent to cooling fan 7 and pulley 6, is curved along the periphery ofcircular locus of cooling fan 7 so as to avoid interfering with coolingfan 7 and pulley 6. Due to external reservoir tank 80 or 180, IHT 1incorporating HST 3, deceleration gearing 50 and differential gearing 60is compacted.

Preferably, the material (such as metal) of housing 2 or aheat-resistant synthetic resin is selected for separating reservoir tank80 or 180 from housing 2. Synthetic resin is advantageous for the easyforming of a reservoir tank having a complicated shape. Alternatively,housing 2 may be partly formed into a reservoir tank having the samefunction as reservoir tanks 80 and 180.

Referring to FIG. 2, reservoir tank 80 has a closed bottom surface inwhich only a vertical port 81 is open. While the bottom surface ofreservoir tank 80 is spaced from the closed top surface of upper housingmember 2 a, port 81 is pointed coaxially to a vertical port 82 bored inthe top wall of upper housing member 2 a so as to bring the interior ofreservoir tank 80 into fluidal communication with the oil sump inhousing 2. Ports 81 and 82 are threaded and a cylindrical bolt 83 isscrewed in between ports 81 and 82. Upper and lower nuts 83 a arescrewed on an external intermediate portion of bolt 83 between reservoirtank 80 and upper housing member 2 a, thereby fastening reservoir tank80 to housing 2. By loosening nuts 83 a, reservoir tank 80 can be easilyremoved from housing 2 so as to facilitate maintenance.

Referring to FIG. 4, reservoir tank 180, serving as an upper half of anoil reservoir, has a substantially entirely open bottom surface. Upperhousing member 2 a is formed with a lower half of the oil reservoirhaving an open top surface. Reservoir tank 180 is mounted on upperhousing member 2 a so as to fit a bottom edge of reservoir tank 180along the open bottom surface onto a top edge of upper housing member 2a along the open top surface, and fastened to upper housing member 2 awith bolts 183. When reservoir tank 180 is completely mounted on housing2, the open bottom surface of reservoir tank 180 coincides with the opentop surface of housing 2, thereby forming an oil reservoir betweenreservoir tank 180 and upper housing member 2 a, which is streamlined incomparison with reservoir tank 80 spaced above housing 2. In otherwords, reservoir tank 180, looking like a part of housing 2, has a clearappearance, which is emphasized if reservoir tank 180 is made of thesame (metal) material with that of housing 2. Reservoir tank 180 may beeasily and economically made of synthetic resin.

Fitting of the inner end of motor shaft 21 into hydraulic motor 20 willnow be described. Firstly, the fitting of motor shaft 21 shown in FIG.1, and then, the fitting of short motor shaft 21 shown in FIGS. 5 to 26will be described.

Motor shaft 21 shown in FIG. 1 is spline-fitted in cylinder block 22.

The inner end portion of motor shaft 21 having the substantially samediameter with that of the spline portion is further extended from thespline portion thereof through cylinder block 22 and slidably rotatablyfitted into a fitting hole of center section 30. Incidentally, asmentioned above, the outer end portion of motor shaft 21 projecting frompartition 2 c is supported between the outer side wall of housing 2 andpartition 2 c so as to be not-relatively rotatably provided thereon withgear 51 and brake disk 74.

Motor shaft 21 shown in FIG. 1 is so axially long as to be fitted intocenter section 30. Further, the extended inner portion of motor shaft 21is diametrically as large as the spline portion thereof. Therefore, thearea of outer peripheral surface of the inner end portion of motor shaft21 frictionally contacting the inner peripheral surface of the fittinghole in center section 30 is considerably large, thereby reducing thepower transmission efficiency from cylinder block 22 to motor shaft 21.

Shortening of motor shaft 21, for reducing or vanishing the peripheralsurface area thereof contacting center section 30, is effective toincrease the efficiency of transmitting power between cylinder block 22and motor shaft 21 and to further minimize HST 3. Although theshortening of motor shaft 21 serving as an input shaft of decelerationgearing 50 causes reduction of durability thereof, there is a room forshortening motor shaft 21 because the gears of deceleration gearing 50,reduced in size or number for reducing the deceleration ratio, can besufficiently driven by shortened motor shaft 21 reduced in durability.However, a new problem arises how cylinder block 22 is surely pivotallysupported on center section 30.

Each of motor shafts 21 shown in FIGS. 5 to 26 is shortened and has aspline portion 21 a serving as the inner end thereof, which isspline-fitted to an axial intermediate portion of cylinder block 22.Namely, the inner end surface of motor shaft 21 is disposed between bothend surfaces of cylinder block 22. Even if motor shaft 21, as shown inFIGS. 13, 14, 21 and 22, has a portion 21 c extended toward centersection 30 from spline portion 21 a, the end surface of the extendedportion 21 c is disposed between both end surfaces of cylinder block 22.Each embodiment of FIGS. 5 to 26 uses means for pivotally supportingcylinder block 22 to center section 30 instead of motor shaft 21.

Shortened and lightened motor shaft 21 has no outer peripheral surfacefrictionally contacting center section 30, thereby enhancing theefficiency of transmitting rotational power from cylinder block 22 tomotor shaft 21 in comparison with motor shaft 21, as shown in FIG. 1,which is so long as to be fitted into center section 30.

Each of hydraulic motors 20 shown in FIGS. 5 to 26, in which the innerend surface of motor shaft 21 is disposed between both end surfaces ofcylinder block 22, will be described.

FIG. 5 shows a basic structure of supporting short motor shaft 21 inhydraulic motor 20. Spline portion 21 a, serving as the inner end ofmotor shaft 21, is spline-fitted to the axial intermediate portion ofcylinder block 22. The end surface of spline portion 21 a (the inner endsurface of motor shaft 21) is spaced from a surface of center section 30fitting cylinder block 22 by an axial hole 22 b of cylinder block 22.Such an arrangement of axial hole 22 b is applied in each of embodimentsshown in FIGS. 6 to 26 excluding FIGS. 17 and 18 in which no axial hole22 b is disposed between the inner end surface of motor shaft 21 and thesurface of center section 30 fitting cylinder block 22 but the axialportion of cylinder block 22 therebetween is solid.

As shown in FIG. 6, a part of spline portion 21 a of motor shaft 21 maybe smoothened on the outer peripheral surface so as to serve as a guideportion 21 b for leading spline portion 21 a into cylinder block 22.Spline portion 21 a of motor shaft 21 shown in each of FIGS. 7 to 26 maybe similarly reshaped.

When motor shaft 21 is supported by cylinder block 22 as shown in FIG. 5(or 6), motor shaft 21 is separated from center section 30, therebycausing the questions of how hydraulic motor 20 is centered to centersection 30, and how cylinder block 22 is slidably and rotatably fittedonto center section 22. The preferred embodiments of FIGS. 7 to 26 givevarious answers.

Referring to FIG. 7, center section 30 is bored by an axial hole, whichis continuously extended from axial hole 22 b of cylinder block 22 whencylinder block 22 is fitted onto center section 30. A thrust bearing 35is fitted across in both of the axial holes of center section 30 andcylinder block 22, that is, slidably abut at the outer peripheralsurface thereof against the inner peripheral surfaces of axial hole 22 band the axial hole of center section 30. Namely, thrust bearing 35serves as means for location of the axis of cylinder block 22 relativeto center section 30, and also serves as a pivot of cylinder block 22relative to center section 30. Thrust bearing 35 shown in FIG. 7 iscylindrical (hollow), however, it may be solid.

Referring to FIG. 8, instead of thrust bearing 35, a solid axial member38 externally fitted with a cylindrical thrust bearing 37 is applied.Axial member 38 has a diametrically large portion fitted into the axialhole of center section 30. The remaining portion of axial member 38fitted into axial hole 22 b of cylinder block 22 is diametricallysmaller than the diametrically large portion. Thrust baring 37 isinterposed between the outer peripheral surface of the diametricallysmaller portion of axial member 38 and the inner peripheral surface ofaxial hole 22 b.

In each of FIGS. 7 and 8, the inner peripheral surface of axial hole 22b of cylinder block 22 slidably abuts against the thrust bearingextended from center section 30. On the other hand, in each of FIGS. 9and 10, the outer peripheral surface of cylinder block 22 partly servesas a slidably abutting surface. Referring to each of FIGS. 9 and 10,center section 30 is formed with an annular projection 30 a rounding theouter peripheral edge of cylinder block 22. Referring to FIG. 9, theinner peripheral surface of annular projection 30 a directly slidablyabuts against the outer peripheral surface of cylinder block 22.Referring to FIG. 10, an annular thrust bearing 36 is interposed betweenannular projection 30 a and the outer peripheral surface of cylinderblock 22.

Referring to each of FIGS. 11 and 12, the inner peripheral surface ofaxial hole 22 b of cylinder block 22 serves as a slide guide surfacewhile center section 30 is partly extended to form a projection 30 binserted into axial hole 22b. Referring to FIG. 11, the outer peripheralsurface of projection 30 b directly slidably abuts against the innerperipheral surface of axial hole 22 b. Referring to FIG. 12, an annularthrust bearing 37 is interposed between the outer peripheral surface ofprojection 30 b and the inner peripheral surface of axial hole 22 b.

Referring to each of FIGS. 13 and 14, center section 30 has projection30 b inserted into axial hole 22 b of cylinder block 22, similar to eachof FIGS. 11 and 12. However, the outer peripheral surface of projection30 b is spaced from the inner peripheral surface of axial hole 22 b(thrust bearing 37 is not even disposed therebetween). Consequently,cylinder block 22 slidably and rotatably abuts against only the verticalflat surface of center section 30. In compensation, motor shaft 21 has adiametrically small end portion 21 c extended toward center section 30from spline portion 21 a and slidably rotatably fitted into projection30 b. Referring to FIG. 13, the outer peripheral surface of extended endportion 21 c of motor shaft 21 directly abuts against the innerperipheral surface of projection 30 b. Referring to FIG. 14, a thrustbearing 39 is interposed between the outer peripheral surface ofextended end portion 21 c and the inner peripheral surface of projection30 b.

Referring to each of FIGS. 15 and 16, cylinder block 22 is relativelyrotatably supported by a part of housing 2. Referring to FIG. 15, theouter peripheral surface of cylinder block 22 directly abuts against thepart of housing 2. Referring to FIG. 16, thrust bearing 36 is interposedbetween the outer peripheral surface of cylinder block 22 and the partof housing 2.

Referring to each of FIGS. 17 and 18, the axial portion of cylinderblock 22 between the inner end surface of motor shaft 21 (the endsurface of spline portion 21 b) and center section 30 is solid (withoutaxial hole 22 b). Cylinder block 22 has an axial projection 22 cinserted into the axial hole bored in center section 30. Referring toFIG. 17, the outer peripheral surface of axial projection 22 c ofcylinder block 22 directly abuts against the inner peripheral surface ofthe axial hole of center section 30. Referring to FIG. 18, annularthrust bearing 37 is interposed between the outer peripheral surface ofaxial projection 22 c and the inner peripheral surface of the axial holeof center section 30.

Referring to each of FIGS. 19 and 20, an axial hole is formed in splineportion 21 a of motor shaft 21, and an axial hole is bored in centersection 20 so as to face the axial hole in spline portion 21 a throughaxial hole 22 b. An axial member 91 is inserted at both ends thereofinto the axial holes of motor shaft 21 and center section 30,respectively, through axial hole 22 b. One end of axial member 91directly slidably abuts at the outer peripheral surface thereof againstthe inner peripheral surface of the axial hole of center section 30.Referring to FIG. 19, the other end of axial member 91 directly slidablyabuts at the outer peripheral surface thereof against the innerperipheral surface of the axial hole of spline portion 21 a of motorshaft 21. Referring to FIG. 20, an annular thrust bearing 92 isinterposed between the outer peripheral surface of the other end ofaxial member 91 and the inner peripheral surface of the axial hole ofspline portion 21 a of motor shaft 21.

Referring to each of FIGS. 21 and 22, an axial member 93 is disposedacross in axial hole 22 b of cylinder block 22 and an axial hole ofcenter section 30 formed coaxially to axial hole 22 b. The outerperipheral surface of axial member 93 directly slidably abuts againstthe inner peripheral surface of the axial hole of center section 30,however, it is spaced from the inner peripheral surface of axial hole 22b. Motor shaft 21 has end portion 21 c extended toward center section 30from spline portion 21 a and inserted into an axial hole formed in aportion of axial member 93 in axial hole 22 b. Referring to FIG. 21, theouter peripheral surface of extended end portion 21 c of motor shaft 21directly slidably abuts against the inner peripheral surface of theaxial hole of axial member 93. Referring to FIG. 22, an annular thrustbearing 94 is interposed between the outer peripheral surface ofextended end portion 21 c and the inner peripheral surface of the axialhole of axial member 93.

Referring to each of FIGS. 23 and 24, surfaces of cylinder block 22 andcenter section 30 slidably rotatably abutting against each other areconcaved and convexed. Referring to FIG. 23, the surface of centersection 30 fitted to motor 20 is convex, and correspondingly, thesurface of cylinder block 22 slidably rotatably abutting against thesurface of center section 30 is concave. Referring to FIG. 24, thesurface of center section 30 fitted to motor 20 is concave, andcorrespondingly, the surface of cylinder block 22 slidably rotatablyabutting against the surface of center section 30 is convex. Even ifhydraulic motor 20 and center section 30 are shaped to fit each other inthis way, the axial location of cylinder block 22 relative to centersection 30 is ensured.

Referring to each of FIGS. 25 and 26, a surface of cylinder block 22facing center section 30 is concaved or convexed, similar to that ofcylinder block 22 of each of FIGS. 23 and 24. However, the verticalsurface center section 30 facing cylinder block 22 is flat, and a convexor concave valve plate 95 or 96 is interposed between the concave orconvex surface of cylinder block 22 and the flat surface of centersection 30. Referring to FIG. 25, a concave surface of cylinder block 22slidably rotatably abuts against the convex surface of valve plate 95.Referring to FIG. 26, a convex surface of cylinder block 22 slidablyrotatably abuts against the concave surface of valve plate 96.

Each of foresaid motor shafts 21, as shown in FIGS. 5 to 26, has an end,which is not extended to center section 30, but is disposed in an axialintermediate portion of cylinder block 22, thereby being shortened so asto reduce load on driving of hydraulic motor 20. Incidentally, cylinderblock 22 with pistons 23 is sandwiched between thrust bearing 25 ofswash plate 24 and center section 30 so as to ensure its slidablyrotatable fitting to center section 30.

Although the invention has been described in its preferred form, with acertain degree of particularity, it is understood that the presentdisclosure of the preferred form has been changed in the details ofconstruction and the combination and arrangement of parts may beresorted to without departing from the spirit and the scope of theinvention as hereinafter claimed.

1. A transaxle, comprising: a housing formed therein with a fluid sump;an axle supported by the housing; a hydrostatic transmission disposed inthe housing, the hydrostatic transmission including an output shaft; abull gear disposed on the axle so as to drivingly connect the outputshaft to the axle; and a reservoir tank removably mounted onto a portionthe housing containing the bull gear, wherein the reservoir tank isfluidly connected to the fluid sump when the reservoir tank is mountedonto the portion of the housing, wherein the reservoir tank includes adownward opening, and the portion of the housing includes an upwardopening, and wherein the upward and downward openings are disposed tocoincide with each other so as to fix the reservoir tank onto theportion of the housing.
 2. The transaxle according to claim 1, furthercomprising: a pipe-shaped joint fixing the reservoir tank onto theportion of the housing.
 3. A transaxle, comprising: a housing formedtherein with a fluid sump; an axle supported by the housing; ahydrostatic transmission disposed in the housing, the hydrostatictransmission including an output shaft; a bull gear disposed on the axleso as to drivingly connect the output shaft to the axle; a reservoirtank removably mounted onto a portion the housing containing the bullgear, wherein the reservoir tank is fluidly connected to the fluid sumpwhen the reservoir tank is mounted onto the portion of the housing,wherein the hydrostatic transmission includes an input shaft projectingoutward from a side of the housing onto which the reservoir tank ismounted; and a cooling fan disposed on the portion of the input shaftprojecting output from the housing, wherein the reservoir tank includesan arcuate wall surrounding the cooling fan.
 4. A transaxle comprising:a housing formed therein with a fluid sump; an axle supported by thehousing; a hydrostatic transmission disposed in the housing, thehydrostatic transmission including an output shaft; a bull gear disposedon the axle so as to drivingly connect the output shaft to the axle; anda reservoir tank removably mounted onto a horizontal surface of a topportion of the housing containing the bull gear so as to be disposedabove the housing when the housing is equipped in a vehicle, wherein thereservoir tank is fluidly connected to the fluid sump when the reservoirtank is mounted onto the horizontal surface of the top portion of thehousing.
 5. The transaxle according to claim 4, further comprising: apipe-shaped joint fixing the reservoir tank onto the top portion of thehousing.
 6. The transaxle according to claim 4, wherein the reservoirtank includes a downward opening, and the top portion of the housingincludes an upward opening, and wherein the upward and downward openingsare disposed to coincide with each other so as to fix the reservoir tankonto the portion of the housing.
 7. A transaxle comprising: a housingformed therein with a fluid sump; an axle supported by the housing; ahydrostatic transmission disposed in the housing, the hydrostatictransmission including an output shaft; a bull gear disposed on the axleso as to drivingly connect the output shaft to the axle; and a reservoirtank removably mounted onto a top portion the housing containing thebull gear when the housing is equipped in a vehicle, wherein thereservoir tank is fluidly connected to the fluid sump when the reservoirtank is mounted onto the portion of the housing, wherein the hydrostatictransmission includes an input shaft projecting outward from a side ofthe housing onto which the reservoir tank is mounted, a cooling fandisposed on the portion of the input shaft projecting output from thehousing, wherein the reservoir tank includes an arcuate wall surroundingthe cooling fan.
 8. A transaxle comprising: a housing formed thereinwith a fluid sump; an axle supported by the housing; a hydrostatictransmission disposed in the housing, the hydrostatic transmissionincluding an output shaft; a bull gear disposed on the axle so as todrivingly connect the output shaft to the axle; and a reservoir tankremovably mounted onto a top portion the housing containing the bullgear when the housing is equipped in a vehicle, wherein the reservoirtank is fluidly connected to the fluid sump when the reservoir tank ismounted onto the portion of the housing, wherein the reservoir tankincludes a downward opening, and the top portion of the housing includesan upward opening, and wherein the upward and downward openings aredisposed to coincide with each other so as to fix the reservoir tankonto the portion of the housing.